The invention generally relates to powering up a bandgap reference circuit.
Bandgap reference circuits are typically chosen due to their ability to produce reference voltages that vary little with temperature. For example, FIG. 1 depicts a typical bandgap reference circuit 10. The circuit 10 includes a high gain operational amplifier 12, three resistors 14, 16 and 17 and two PEP bipolar junction transistors (BATS) 18 and 20.
Regarding the specific structure of the bandgap reference circuit 10, the output terminal of the amplifier 12 provides a bandgap reference voltage (called xe2x80x9cVbgxe2x80x9d). Each BJT 18 and 20 has its base terminal coupled to its collector terminal, and the collector terminal of each BJT 18, 20 is coupled to ground. The emitter terminal of the BJT 18 is coupled to the output terminal of the amplifier 12 through the resistors 14 and 17. The emitter terminal of the BJT 20 is coupled to the output terminal of the amplifier 12 through the resistor 16. The inverting input terminal of the amplifier 12 is coupled to a node between the resistors 14 and 17, and the non-inverting input terminal of the amplifier 12 is coupled to the emitter terminal of the BJT 20. As depicted in FIG. 1, a current called I1 flows through the emitter-collector path of the BJT 18, and a current called I2 flows through the emitter-collector path of the BJT 20.
Due to the high gain of the amplifier 12, the non-inverting and inverting input terminals of the amplifier 12 are approximately equal to establish the following relationship:
Vbe1+I1*R3=Vbe2,xe2x80x83xe2x80x83Equation 1
where xe2x80x9cVbe1xe2x80x9d and xe2x80x9cVbe2xe2x80x9d are the base-emitter voltages of the BATS 18 and 20, respectively, and xe2x80x9cR3xe2x80x9d represents the resistance of the resistor 17. From this relationship, the I1 current may be calculated as described below:
I1=(Vbe2xe2x88x92Vbe1)/R3xe2x80x83xe2x80x83Equation 2
If it is assumed that the resistors 14 and 16 have the same resistances, then the I2 current equals the I1 current, and from Equations 1 and 2, the Vbg bandgap reference voltage may be calculated as described below:
Vbg=Vbe1+(1+R1/R3)*(Vt*ln(n)),xe2x80x83xe2x80x83Equation 3
where xe2x80x9cVtxe2x80x9d is the thermal voltage that is equal to approximately 25.875 mV at room temperature, xe2x80x9cnxe2x80x9d is the ratio of the areas of the BATS 18 and 20 and xe2x80x9cR1xe2x80x9d is the resistance of the resistor 14, 16.
In Equation 3, the Vbel voltage has a negative proportional-to-absolute-temperature (PTAT) coefficient, and the second term on the right-hand side of the equation has a positive PTAT. Therefore, by controlling the ratio of the resistances 14 and 17 and the ratio n, the Vbg bandgap reference voltage may have very little dependency on temperature.
However, a potential difficulty with the bandgap reference circuit 10 is that there are two possible solutions for Vbg in Equation 3. Thus, the Vbg bandgap reference voltage may be either a well-controlled voltage (1.25 volts, for example) as desired, but the Vbg voltage may also be zero volts. For example, a scenario in which the Vbg bandgap reference voltage is zero volts may occur due to the circuit 10 being powered down, a state of the circuit 10 in which the Vbg bandgap reference voltage is zero volts. When the bandgap reference circuit 10 powers up and transitions into its normal mode of operation, however, the Vbg bandgap reference voltage may not change from zero volts.
Referring to FIG. 2, to prevent the above-described scenario from occurring, a start-up circuit, such as a start-up circuit 30 that is depicted in FIG. 2, typically accompanies the bandgap reference circuit 10 and is used for the purpose of ensuring that the Vbg bandgap reference voltage indicates the desired solution to Equation 3. The start-up circuit 30 may include several resistors, such as an explicit resistor 32 and n-channel metal-oxide-semiconductor field-effect-transistors (NMOSFETs) 34, 36 and 38 that are configured as resistors. These resistors form a resistor divider to scale down a supply voltage (called Vcc) to provide a voltage and a current to the emitter terminal of the BJT 20. Due to this arrangement, when the bandgap reference circuit 10 powers up, current flows through the emitter-collector path of the BJT 20 to produce a nonzero voltage at the non-inverting input terminal of the amplifier 12. This voltage, in turn, produces a nonzero voltage at the inverting input terminal of the amplifier 12 if the input voltage swing of the amplifier 12 is sufficient. Thus, non-zero voltages and currents that are produced by the start-up circuit 30 should ideally prevent the Vbg bandgap reference voltage from being zero volts after power up.
There are potential drawbacks to the start-up circuit 30. For example, the amplifier 12 may not operate correctly if the Vbe2 voltage is too low, thereby causing the Vbg bandgap reference voltage to still come up at zero volts. Furthermore, the start-up circuit 30 consumes current during the normal mode of operation of the bandgap reference circuit 10, after the power-up has been completed. This may be disadvantageous if the bandgap reference circuit 10 is used in, for example, a wireless or portable product that requires low power operation.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.